Chlorine dioxide is a valuable chemical, typically used in aqueous solution form in the bleaching of cellulosic fibrous material pulps, and is known to be formed in a number of ways, generally involving the reduction of a chlorate by chloride in an acid medium.
The basic reaction involved in such processes is summarized by the equation: EQU ClO.sub.3.sup.- + Cl.sup.- + 2H.sup.+ .fwdarw. ClO.sub.2 + 1/2 Cl.sub.2 + H.sub.2 O (1)
commonly, the chlorate ions are provided by sodium chlorate, the chloride ions by sodium chloride and/or hydrogen chloride and the hydrogen ions by sulphuric acid and/or hydrochloric acid. Along with the reaction depicted by equation (1), there is a competing reaction which decreases the efficiency of the conversion of chlorate ions to chlorine dioxide, represented by the following equation 2: EQU ClO.sub.3.sup.- + 6H.sup.+ + 5Cl.sup.- .fwdarw. 3Cl.sub.2 + 3H.sub.2 O (2)
in one known manner of producing chlorine dioxide, as set forth in more detail in Canadian Pat. No. 826,577, the aqueous reaction medium containing sodium chlorate, sodium chloride and/or hydrogen chloride and sulphuric acid at an acidity of about 2 to 4.8N, is maintained at its boiling point while the reaction vessel is maintained under a subatmospheric pressure, resulting in evaporation of water from the reaction medium, removal of the chlorine dioxide and chlorine in gaseous admixture with the steam and deposition of solid by-products, mainly anhydrous sodium sulphate, in the reaction vessel. The reaction is carried out at an elevated temperature below that above which substantial decomposition of chlorine dioxide would occur. The gaseous mixture of chlorine dioxide, chlorine and steam then is processed to condense the steam and recover an aqueous solution of chlorine dioxide, which usually contains some dissolved chlorine.
The sodium sulphate deposited as by-product is removed from the reaction vessel, continuously or intermittently, and is utilized as make-up chemical in the recovery cycle of a pulp mill of any convenient type, typically a Kraft pulp mill.
In a Kraft pulp mill operation, wood chips or other raw cellulosic fibrous material are digested with a white liquor containing sodium sulphide and sodium hydroxide as the active pulping hemicals to form a wood pulp which is separated from spent pulping liquor, otherwise known as black liquor.
The pulp thereafter usually is subjected to bleaching and purification operations in a bleach plant to form a pulp of desired brightness, the bleaching steps typically utilizing the chlorine dioxide solution produced in the chlorine dioxide-producing system.
The black liquor is subjected to recovery and regeneration operations to form fresh white liquor for recycle to the digestion stage to provide at least part of the white liquor used therein. The recovery and regeneration operations generally involve an initial concentration of the black liquor followed by burning in a furnace to form a smelt containing sodium sulphide and sodium carbonate. The smelt is dissolved in water to form an aqueous solution thereof, known as green liquor, which, after clarification to remove undissolved solids, is causticized with slaked lime resulting in the conversion of carbonate to hydroxide, with consequent deposition of calcium carbonate. The calcium carbonate is recovered and usually is used to form further slaked lime. The aqueous solution resulting from separation of the calcium carbonate therefrom is the recycled white liquor.
The sodium sulphate recovered from the chlorine dioxide generator is used as a source of make-up chemicals for this system, to make up losses of soda and sulphur values. The quantity of make-up chemical required varies from mill to mill, although in most cases it is less than the chlorine dioxide requirement, due to tightening up of the mill recovery system, with less losses thus being experienced
The above-mentioned chlorine dioxide-producing procedure provides about 1 mole of sodium sulphate and 1/2 to 1 mole of chlorine for each mole of chlorine dioxide formed. With the decrease in losses of sodium and sulphur values, the prior art process typically produces more sodium sulphate than can be used by the mill and hence the excess accumulates.
Further, with the tendency to use less quantities of chlorine in bleaching, excess quantities of chlorine gas may be produced.